Amplifiers such as class A amplifiers and non-linear amplifiers are often used to amplify signals which have a constant envelope. Linear amplifiers have low efficiency, whereas non-linear amplifiers that operate in switching mode are highly efficient. Various conventional amplifiers have been proposed in attempts to achieve linear amplification using non-linear amplifiers instead of linear amplifiers. Examples of such conventional amplifiers are Kahn envelope elimination and restoration (EER) amplifiers, Pulse Width Modulation (PWM) amplifiers and linear amplification with nonlinear components (LINC) amplifiers.
FIG. 1 shows an EER amplifier 100. The EER amplifier 100 includes an envelope modulator 110, a driver 120 and a Radio Frequency (RF) power amplifier 130. Both the envelope modulator 110 and the RF power amplifier have high efficiencies.
A linear modulation signal is separated into a variable envelope modulation signal E(t) and a constant envelope RF phase modulated signal X(t), whereX(t)=cos(ωc(t)+θ(t)).
The variable envelope modulation signal E(t) and the constant envelope RF phase modulated signal X(t) are processed, respectively, by the envelope modulator 110 and the driver 120 of the EER amplifier 100 and subsequently combined. More specifically, the envelope modulator 110 modulates supply voltage provided to the RF power amplifier 130, which amplifies the constant envelope RF phase modulated signal X(t) via the driver 120.
Although the EER amplifier 100 is highly efficient, its linearity is limited by the bandwidth of the envelope modulator 110, the alignment of the processed variable envelope modulation signal E(t) and the processed constant envelope RF phase modulated signal X(t). Moreover, the envelope modulator 110 requires a low pass filter, which is bulky and is hence not suitable for integration with other systems.
FIG. 2 shows a PWM amplifier 200, which is constructed using a bandpass delta-sigma modulator 210, a class-S power amplifier 220 (which is an example of a highly efficient non-linear power amplifier) and a bandpass filter 230. The PWM amplifier 200 modulates the pulsewidth, or pulse density, of a constant envelope signal that is provided to the bandpass delta-sigma modulator 210. The bandpass delta-sigma modulator 210 then generates a PWM signal which is provided to the class-S power amplifier 220 and amplified. The bandpass filter 230 filters out-of-band noises from the class-S power amplifier 220.
The PWM signal is, for example, a single bit digital data stream, therefore the operating clock frequency of the bandpass delta-sigma modulator 210 needs to be high. More specifically, the operating clock frequency needs to be a few times higher, typically four times higher, than that of the central frequency of interest. Hence, the bandwidth of the PWM amplifier 200 is limited.
As the bandpass delta-sigma modulator 210 needs to be operated at high operating clock frequencies, the application of the PWM amplifier 200 for constant envelope signals within higher RF frequency ranges is a problem. Furthermore, at high operating clock frequencies, power consumption of the bandpass delta-sigma modulator 210 is increased. Therefore, the PWM amplifier 200 is not suitable for use in applications where wideband transmission is required. Additionally, the bandpass filter 230 increases size and cost of the PWM amplifier 200. Moreover, amplified signals provided by the class-S power amplifier 220 may also be attenuated by the bandpass filter 230.
FIG. 3a-b shows a LINC amplifier 300. As shown on FIG. 3a, the LINC amplifier 300 has a LINC Signal generator (LSG) 310, a first power amplifier 320, a second power amplifier 330 and a combiner 340, which is a passive combiner.
The LSG 310 receives input signals S(t) and transforms the input signals S(t) a first signal component S1(t) and a second signal component S2(t), each of which are nonlinear constant envelope signals. Each of the first and second power amplifiers 320/330 has an input and an output coupled to the LSG 310 and the combiner 340, respectively. The first and second signal components S1(t)/S2(t) are provided to the first and second power amplifiers 320/330, respectively, and amplified, before being provided to the combiner 340. The combiner 340 receives the amplified first and second signal components, which are denoted by symbols GS1(t) and GS2(t), respectively, and combines them to produce a linear output signal GxS(t).
Outputs of each of the first and second power amplifiers 320/330 are required to be matched in gain and phase by approximately 0.5 dB and 0.3° respectively. Such requirements are extremely difficult to meet without the use of a complex feedback system.
The efficiency of the LINC amplifier 300 is dependent on the efficiency of first and second power amplifiers 320/330 and the combiner 340. Therefore, the efficiency of the combiner 340 is critical since the first and second power amplifiers 320/330 are high efficient nonlinear amplifiers. The efficiency of the combiner 340 is dependent on the type of combiner used, which is typically an isolated combiner or a Chireix-outphasing combiner. The isolated combiner provides a linear output, however its efficiency is limited. On the other hand, the Chireix-outphasing combiner provides better efficiency but suffers from linearity issues.
The LINC amplifier 300 operates based on vector summing of the phase α(t) of the first and second signal components S1(t)/S2(t) as shown in FIG. 3b. 
It is therefore desirable to provide a solution for addressing at least one of the foregoing problems of such conventional amplifiers.